This application claims the benefit of Japanese Application No. 2002-330665 filed Nov. 14, 2002.
The present invention relates to a static magnetic field generating apparatus and a magnetic resonance imaging apparatus employing permanent magnets supported by columnar yokes and base yokes forming a magnetic circuit.
In recent years, magnetic resonance imaging apparatuses in which a static magnetic field is generated by permanent magnets have become widespread. Because high accuracy is required in such apparatuses, a technique of adjusting the static magnetic field has been applied mainly by using iron pieces or the like on the permanent magnets themselves or on base yokes attached with the permanent magnets.
Another technique involves winding coils around columnar yokes for supporting the permanent magnets to electrically generate magnetic flux, thereby correcting the static magnetic field (see Patent Document 1, for example).
Patent Document 1
Japanese Patent Application Laid Open No. 2002-238872 (page 1, FIG. 10).
The conventional techniques, however, did not allow easy adjustment of the static magnetic field. Specifically, the technique of adjusting the static magnetic field on the permanent magnets themselves or on the base yokes attached with the permanent magnets must be completed in the assembling process of the magnetic resonance imaging apparatus on account of the weight of the permanent magnets, for example. On the other hand, the technique of electrically correcting the static magnetic field requires extra instruments because coils and a control device therefor are used.
Especially, the permanent magnet is more vulnerable to environmental conditions such as temperature change than a superconductive magnet, and the fact that the adjustment for maintaining accuracy of the static magnetic field cannot be easily achieved at an installation site such as a hospital, is not preferable from the viewpoint of performance and servicing efficiency.
It is therefore important to find a way to realize a static magnetic field generating apparatus and a magnetic resonance imaging apparatus employing permanent magnets that allow easy adjustment of a static magnetic field.
An object of the present invention is to provide a static magnetic field generating apparatus and a magnetic resonance imaging apparatus employing permanent magnets that allow easy adjustment of a static magnetic field.
For solving the aforementioned problems and attaining this object, a static magnetic field generating apparatus, in accordance with a first aspect of the invention, is characterized in comprising: a pair of permanent magnets opposingly disposed across a space in which a subject is placed; a pair of base yokes for supporting said permanent magnets; and columnar yokes for magnetically connecting said base yokes and structurally supporting them, said columnar yokes having magnetic resistance modifying means.
In the invention of the first aspect, permanent magnets are opposingly disposed across a space in which a subject is placed, the permanent magnets are supported by a pair of base yokes, and the base yokes are magnetically connected and structurally supported by columnar yokes having magnetic resistance modifying means; and therefore, magnetic flux generated in the space by the permanent magnets, hence, a static magnetic field that is equal to magnetic flux per unit area, can be easily changed and adjusted by modifying magnetic resistance of the columnar yokes.
The static magnetic field generating apparatus, in accordance with a second aspect of the invention, is characterized in that: said modifying means comprises a groove on a side surface of at least one of said columnar yokes, said side surface lying in a longitudinal direction of said columnar yoke.
In the invention of the second aspect, since a groove is provided on a side surface of the columnar yoke lying in a longitudinal direction of the columnar yoke as the modifying means, the cross-sectional area of the columnar yoke perpendicular to the longitudinal direction is reduced at the groove, thereby increasing magnetic resistance.
The static magnetic field generating apparatus, in accordance with a third aspect of the invention, is characterized in that: said groove has a rectangular cross section.
In the invention of the third aspect, since the groove has a rectangular cross section, it can be easily machined.
The static magnetic field generating apparatus, in accordance with a fourth aspect of the invention, is characterized in that: said modifying means comprises an insert member of a shape generally conforming to said groove.
In the invention of the fourth aspect, since an insert member of the modifying means has a shape generally conforming to the groove, magnetic resistance at the groove can be modified by inserting/removing the insert member into/from the groove.
The static magnetic field generating apparatus, in accordance with a fifth aspect of the invention, is characterized in that: said modifying means comprises a through hole passing through a side surface of at least one of said columnar yokes, said side surface lying in parallel with a longitudinal axis of said columnar yoke.
In the invention of the fifth aspect, since a through hole is provided to pass through a side surface of the columnar yoke lying in parallel with a longitudinal axis of the columnar yoke as the modifying means, the cross-sectional area of the columnar yoke perpendicular to the longitudinal axis is reduced at the through hole, thereby increasing magnetic resistance.
The static magnetic field generating apparatus, in accordance with a sixth aspect of the invention, is characterized in that: said through hole has a circular cross section.
In the invention of the sixth aspect, since the through hole has a circular cross section, reduction in strength of the columnar yokes for supporting the permanent magnets can be mitigated.
The static magnetic field generating apparatus, in accordance with a seventh aspect of the invention, is characterized in that: said modifying means comprises a filling member of a shape generally conforming to said through hole, said filling member being insertable/removable into/from said columnar yoke.
In the invention of the seventh aspect, since the modifying means comprises a filling member of a shape generally conforming to the through hole to be insertable/removable into/from the columnar yoke, magnetic resistance at the through hole can be modified by inserting/removing the filling member into/from the through hole.
The static magnetic field generating apparatus, in accordance with an eighth aspect of the invention, is characterized in that: said modifying means comprises a threaded hole provided on a side surface of at least one of said columnar yokes, said side surface lying in parallel with a longitudinal axis of said columnar yoke.
In the invention of the eighth aspect, since the modifying means comprises a threaded hole provided on a side surface of the columnar yoke lying in parallel with a longitudinal axis of the columnar yoke, the cross-sectional area of the columnar yoke perpendicular to the longitudinal axis is reduced at the threaded hole, thereby increasing magnetic resistance.
The static magnetic field generating apparatus, in accordance with a ninth aspect of the invention, is characterized in that: said modifying means comprises a screw of a shape generally conforming to said threaded hole, said screw being insertable/removable into/from said columnar yoke.
In the invention of the ninth aspect, since a screw of a shape generally conforming to the threaded hole serving as the modifying means is inserted/removed into/from the columnar yoke, magnetic resistance at the threaded hole can be modified by inserting/removing the screw into/from the threaded hole.
The static magnetic field generating apparatus, in accordance with a tenth aspect of the invention, is characterized in that: said groove, said through hole or said threaded hole is disposed on the side surface of said columnar yoke on the side where said space lies.
In the invention of the tenth aspect, since the groove, through hole or threaded hole is disposed on the side surface of the columnar yoke on the side where the space lies, magnetic flux leaking from the groove, through hole or threaded hole is efficiently returned to the permanent magnets, thereby efficiently modifying magnetic flux generated from the permanent magnets.
The static magnetic field generating apparatus, in accordance with an eleventh aspect of the invention, is characterized in that: said groove, said through hole or said threaded hole is disposed in a bending portion at which said permanent magnet and said columnar yoke are joined.
In the invention of the eleventh aspect, since the groove, through hole or threaded hole is disposed in a bending portion at which the permanent magnet and columnar yoke are joined, magnetic flux in the bending portion where magnetic flux is concentrated can be efficiently adjusted by the groove, through hole or threaded hole.
The static magnetic field generating apparatus, in accordance with a twelfth aspect of the invention, is characterized in that: said modifying means comprises a different-material portion provided within at least one of said columnar yokes and composed of a material having a different magnetic permeability from that of said columnar yoke.
In the invention of the twelfth aspect, since the modifying means comprises a different-material portion provided within the columnar yoke and composed of a material having a different magnetic permeability from that of the columnar yoke, magnetic resistance of the columnar yoke can be modified by a large amount for adjustment.
The static magnetic field generating apparatus, in accordance with a thirteenth aspect of the invention, is characterized in that: said different-material portion has a cross-sectional shape identical to a cross section perpendicularly intersecting the longitudinal axis of said columnar yoke.
In the invention of the thirteenth aspect, since the cross-sectional shape of the different-material portion is identical to a cross section perpendicularly intersecting the longitudinal axis of the columnar yoke, leakage of magnetic flux can be minimized.
The static magnetic field generating apparatus, in accordance with a fourteenth aspect of the invention, is characterized in that: a plurality of said groove, said through hole, said threaded hole or said different-material portion are disposed at symmetric positions of the respective columnar yokes with respect to the position of the subject.
In the invention of the fourteenth aspect, since a plurality of the groove, the through hole, the threaded hole or the different-material portion are disposed at symmetric positions of the respective columnar yokes with respect to the position of the subject, magnetic resistance can be equalized between the columnar yokes, and magnetic flux generated in the space can be homogenized without directionality.
A magnetic resonance imaging apparatus, in accordance with a fifteenth aspect of the invention, comprises: a static magnetic field generating apparatus for generating a static magnetic field using permanent magnets; gradient magnetic field generating means for generating a gradient magnetic field; transmitting/receiving means for transmitting/receiving a radio frequency magnetic field in said static magnetic field; and a control section for controlling said gradient magnetic field generating means, said transmitting means and said receiving means, said magnetic resonance imaging apparatus being characterized in that said static magnetic field generating apparatus comprises, in columnar yokes that magnetically connect and structurally support base yokes supporting a pair of opposingly disposed said permanent magnets, modifying means for modifying magnetic resistance of said columnar yokes.
In the invention of the fifteenth aspect, the static magnetic field generating apparatus comprises, in columnar yokes that magnetically connect and structurally support base yokes supporting a pair of opposingly disposed permanent magnets, modifying means for modifying magnetic resistance of the columnar yokes; and therefore, magnetic flux generated in the space by the permanent magnets, hence, a static magnetic field that is equal to magnetic flux per unit area, can be easily changed and adjusted by modifying magnetic resistance of the columnar yokes.
The magnetic resonance imaging apparatus, in accordance with a sixteenth aspect of the invention, is characterized in that: said modifying means comprises a groove on a side surface of at least one of said columnar yokes, said side surface lying in parallel with a longitudinal axis of said columnar yoke, a through hole passing through said side surface, or a threaded hole provided on said side surface; and an insert member of a shape generally conforming to said groove, a filling member of a shape generally conforming to said through hole, or a screw of a shape generally conforming to said threaded hole.
In the invention of the sixteenth aspect, the modifying means comprises a groove on a side surface of the columnar yoke lying in parallel with a longitudinal axis of the columnar yoke, a through hole passing through the side surface, or a threaded hole on the side surface, and an insert member of a shape generally conforming to the groove, a filling member of a shape generally conforming to the through hole, or a screw of a shape generally conforming to the threaded hole; and therefore, magnetic flux in the space can be easily adjusted with high accuracy by inserting/removing the insert member, filling member and screw into/from the groove, through hole and threaded hole.
According to the present invention, permanent magnets forming a pair are opposingly disposed across a space in which a subject is placed, the permanent magnets are supported by a pair of base yokes, and the permanent magnets are magnetically connected and structurally supported by columnar yokes having magnetic resistance modifying means; and therefore, magnetic flux generated in the space by the permanent magnets, hence, a static magnetic field that is equal to magnetic flux per unit area, can be easily modified and adjusted by changing magnetic resistance of the columnar yokes.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.